So it looks like the cap is actually trunc(50% of idea groups) +1, not 50% of idea groups. Dear god I'm tired. The cap is just rounded up. i.e...
1 max at 1-2 total
2 max at 3-4 total
3 max at 5-6 total
4 max at 7-8 total
Which is a bit more manageable, and frankly, I can't think of a SP situation in which I'd want to break that -- or have broken it, IIRC. At least since Exploration and Expansion were both DIP and the first two picks for a couple starts.
That being said, what a bizarre design decision. Opportunity should be important enough of a resource to ward away from bunching up on idea groups from a single category; if it isn't, that's a problem with the balance of the categories.